First there was Gregor Mendel, a monk who studied
inherited characteristics. This was followed by Francis Crick and James
Watson who unraveled the DNA molecule. This has led us to uderstanding
the Human Genome sequence. This section will examine and document the
events leading up to this remarkable achievement.

DNA Structure:A rotating, wire frame model of a Watson-Crick DNA double helix. The
two chains run anti-parallel to each other and contain matched pairs of
nitrogenous bases: adenine with thymine (A-T), guanine with cytosine (G-C).
This complementary structure allows for two identical daughter molecules
to be made from a single parent molecule and is termed semiconservative
replication, meaning one strand of the new DNA molecule comes from the
original DNA.

DNA Replication:In this animation, adenine, which is green, is the complimentary base
pair for thymine, which is pink. Guanine, which is yellow forms a complement
with cytosine, which is blue. Note the specific way in which the different
nitrogenous bases are paired. The adenine-thymine (A-T for short) pair
is formed by two hydrogen bonds and the guanine-cytosine (G-C) base pair
has 3 hydrogen bonds. Adenine can only bond with thymine and vice-versa.
In the same fashion, guanine and cytosine can only bond with each other.
In this way, each strand is the compliment of the other and allows for
the žsemiconservativeÓ of DNA replication.

Cell & Nucleus:Here we have a diagram of a typical cell from the human body. In this
picture you can get a sense of where the DNA resides in the cell as well
as how it is organized in the nucleus. Double-stranded DNA is organized
into chromosomes. Chromosomes are situated in the nucleus and the membrane
bound nucleus is found in the cell.

Each individual is identified by a relatively unique
combination of nucleotides found in long coiled strands of DNA organized
as chromosomes found in a cells nucleus. The number and arrangement
of chromosomes in an organism is characteristic of that organism,
and can be represented as a karyotype, which derived by arranging
the chromosomes in pairs by size. The karyotype can be used to show
differences in genetic makeup (genotype), which determines the features
that person has (phenotype). Karyotypes of various humans are shown.

Normal Male

Shows 23 pairs of chromosomes. Twenty two pairs are called autosomes,
1 pair is called the sex chromosomes ; XY in a male. Males are associated
with secondary sexual characteristics  abundant facial hair, voice,
others. Differences are also evident in the genitalia.

Normal Female

Shows 23 pairs as in the male, but sex chromosomes are represented
by XX. Females also have different physical features from males. A Barr
body is also present in cells of the female, representing an inactivated
X chromosome.

Klinefelter's Syndrome

Three sex chromosomes are associated with Klinefelter rather than
the expected 2 - XXY. These individuals are males with some development
of breast tissue normally seen in females. Little body hair is present,
and such person are typically tall, with or without evidence of mental
retardation. Males with XXXY, XXXXY, and XXXXXY karyotypes have a more
severe presentation, and mental retardation is expected.

Normally associated with 3 copies of chromosome number 21(trisomy
of chromosome 21), rather than the 2 found normally. Down syndrome is
characterized by differing degrees of mental retardation, a skin fold
over the eye, typically short stature, and short hands with a deep crease
in the palm. Down is also known as mongolism (mongoloid).